A continuing need for increased data capacity in wired and wireless broadband data networks is being driven by growth in support applications such as on-demand, hi-definition television, cloud services, mobile applications and the like. This demand urges the development of new data capacity for such wired and wireless systems. Thus, there is a need to provide a more efficient and robust use of new or existing networks that use intensity modulation (IM) as a means of transmission and reception and which may use orthogonal frequency division multiplexing (OFDM). OFDM may be used in fiber optic communications, in free space optical wireless communications (OWC) systems (including Visible Light Communications (VLC)), and acoustic communications. OFDM represents one class of IM approaches. Fiber optic systems using IM are commonly found in broadband data distribution and access networks serving businesses, campuses, consumer homes, and the like. VLC, a form of OWC which uses IM, is a communications technique employing visible light in a free-space model and targets delivery of data from lights (luminaires) as wireless access points. Other forms of OWC that are applicable to this invention include other light spectra including infra-red and ultraviolet.
OFDM can achieve the needed robust high-capacity links discussed above because it offers high spectral efficiency, adequate bit loading, and power allocation per sub-carrier and simple frequency domain equalization. High data rates are supported through parallel transmission of high order multi-level quadrature amplitude modulation (QAM) symbols on orthogonal sub-carriers. In intensity modulation (IM) transmission using OFDM, the real-valued base band OFDM signals modulates the instantaneous power of the optical carrier that is detected using a photo detector.
Conventional radio frequency OFDM (RF-OFDM) signals are complex. That is, they include both real and imaginary parts. Therefore RF-OFDM signals cannot be directly used to modulate the instantaneous power of the optical carrier because only real and positive signals can be used to support intensity modulation.
There are several conventional solutions to address the problem of needing to generate real and positive signals from a complex OFDM signal in optical transmission. One solution, known as DC-biased optical OFDM (DCO-OFDM) creates real and bi-polar symbols and introduces a DC bias to shift bipolar symbols so that all the values of the symbol are positive. However, DCO-OFDM sacrifices 50% of the conventional RF-OFDM spectral efficiency because the quadrature amplitude modulation (QAM) constellations to the Inverse Fast Fourier Transform (IFFT) engine are constrained by Hermitian symmetry.
The bipolar nature of OFDM signals introduces an additional problem in VLC because LEDs can only convey unipolar signals in light intensity. A power efficient alternative to DCO-OFDM is asymmetrically clipped optical OFDM (ACO-OFDM) which eliminates DC biasing. ACO-OFDM uses properties of the Fourier Transform and asymmetrical clipping to create real and unipolar symbols in the time domain. Because ACO-OFDM uses only positive samples, it has one-half the spectral efficiency of DCO-OFDM and one-fourth the spectral efficiency of RF-OFDM.
The DCO-OFDM signal is real-valued and bipolar while the ACO-OFEM signal is real-valued and unipolar. However, some proportion of the spectral efficiency must be sacrificed to ensure real-valued signals from complex signals. The IFFT engine is the main operation to modulate orthogonal sub-carriers and generate time-domain OFDM samples used to modulate the LED intensity in IM. As discussed above, the spectral efficiency of DCO-OFDM is one-half of the spectral efficiency of RF-OFDM and the spectral efficiency ACO-OFDM is one-half the spectral efficiency of DCO-OFDM or one-fourth the spectral efficiency of RF-OFDM. This is because both of these techniques rely on a complex conjugate assignment, also called Hermitian symmetry, to get real values out of the QAM complex samples input to the IFFT engine. With Hermitian symmetry, half of the available sub-carriers are used to carry complex data QAM symbols while the other half carries redundant complex-conjugate of the complex data QAM symbols. In addition to Hermitian symmetry, ACO-OFDM uses only odd sub-carriers to carry data—the QAM constellations are assigned only to odd sub-carriers while even sub-carriers are assigned a zero value.
Other conventional techniques to generate unipolar OFDM symbols, such as Flip-OFDM (F-OFDM), position modulation OFDM (PM-OFDM), unipolar OFDM (U-OFDM), and pulse amplitude modulation discrete multitone (PAM-DMT) similarly have one-fourth the spectral efficiency of RF-OFDM.